In the construction field it is now common to install a gypsum-based sub-floor, or underlayment, over new or existing floor structures, i.e., wood planking, plywood, structural concrete, etc. This is done by formulating a fluid wet mix and pouring, pumping or spreading it over the existing floor. The benefits of this technique are numerous. First, (and particularly when the wet gypsum-based composition is applied over wood, e.g., plywood floors) the wet mix levels itself to a large extent, providing a surface which is free from splinters, squeaks and nail heads. As such, floor coverings such as linoleum, wood parquet, vinyl, or carpeting may easily be laid over the floor after it sets. Second, because the wet mix is fluid, it fills in cracks in the floor and in perimeter joints, i.e., wall-floor interfaces. The floor is thus stiffened, and sound leaks between rooms and stories are consequently greatly reduced. Third, because the composition is predominantly gypsum-based, it retards the spread of flame and smoke to adjoining areas. Fourth, the pourable sub-floor composition may be used in rehabilitating cracked, worn, low-to-medium traffic concrete floors; by applying the composition over the existing floor, the fluid composition fills the cracks and levels out any depressions or elevations in the floor.
A method for providing a higher strength hardened floor having a compressive strength on the order of 2500-6000 psi, and sub-floors meeting those specifications, have long been desired, however. Methods for sub-floor preparation taught, for example, in U.S. Pat. Nos. 4,075,374, 4,159,912, and 4,444,925, issued to Jorgensen et al., Jorgensen, and Feldman, respectively, do not provide floors having the requisite strength. In particular, floors obtained by the '374 patent were found to be unsuitable for actual use. In the '912 patent, the floors had a tendency to develop a dusty or dust-like surface. This required treatment of the surface in some instances, e.g., cleaning and sealing, before any floor covering could be properly applied. Furthermore, the hardened sub-floors made according to the '374 patent had a predisposition to delaminating from the floor over which the wet composition was poured.
It is therefore an object of this invention to provide a method for preparing a gypsum-based sub-floor having high compressive strengths, little or no surface dusting, resistance to cracking, and resistance to delamination from the floor beneath the sub-floor.